A typical power amplifier does not behave linearly. There are occasions when the power amplifier gives compression to the output of the amplifier and there are occasions when it gives expansion to the output. Typical signal detectors that receive and decode these amplified signals cannot operate in such a non-linear fashion. Therefore, it is necessary to linearize the amplifier output by applying inverse distortion at the input to the power amplifier to undo the compression or expansion produced by the amplifier. Digital pre-distorters are commonly used with power amplifiers to invert the power amplifier saturation characteristics by expanding the saturation regions and compressing the expansion regions in the power amplifier characteristics curve.
FIG. 1 is a block diagram of a typical digital pre-distorter 1 and power amplifier 2. The input signal received by the pre-distorter 1, VIN, is pre-distorted by the pre-distorter 1 into a pre-distorted signal, VPD. The pre-distorted signal VPD is such that the non-linearities of the power amplifier 2 cause the power amplifier 2 to produce an output signal, VOUT—TARGET, that is closer to what an ideal output signal of the power amplifier 2 should be.
The basic principles of a typical digital pre-distorter can be seen from the graph shown in FIG. 2. In FIG. 2, the vertical axis represents the voltage output of a typical power amplifier and the horizontal axis represents the voltage input to the power amplifier. The linear operation of the power amplifier is represented by the following equation:y=mx+c,where m is the linear gain of the power amplifier and c=0 is the output intercept point. This equation corresponds to line 3 in FIG. 2, which has a slope equal to m. The curve 4 in the graph of FIG. 2 corresponds to a non-pre-distorted output characteristic curve for a typical power amplifier. The non-linearities of the power amplifier result in amplitude-to-amplitude (AM/AM) distortion as well as amplitude-to-phase (AM/PM) distortion, which results in curve 4 being non-linear.
A digital pre-distorter actually increases or decreases the amplifier input magnitude to linearize the amplifier output, i.e., to make curve 4 look more like line 3. The digital pre-distorter asserts a negative phase distortion to mitigate the phase distortion introduced by the power amplifier. It can be seen from FIG. 2 that in order to increase the amplifier output magnitude by ΔVOUT so that Vout_target=Vout_nopd+ ΔVout, the amplifier input magnitude needs to be increased by ΔVIN so that Vpd=Vin+Δ Vin. The pre-distorter performs these functions by first fitting an odd ordered polynomial to the power amplifier magnitude characteristic curve (e.g., curve 4 in FIG. 2), calculating the inverse of the polynomial, and then applying the coefficients of the inverse polynomial to the amplifier input in the pre-distorter to linearize the amplifier output. In essence, applying the coefficients to the amplifier input creates inverse distortion that reverses any compression or expansion caused by the amplifier.
Furthermore, the amplifier output characteristics change over short and long periods of time giving rise to what are commonly referred to as slow memory effect and fast memory effect. Therefore, the appropriate polynomial must be selected for the appropriate circumstances. Then, its inverse polynomial obtained, and then the coefficients of the inverse polynomial applied to the amplifier input to cause the amplifier output characteristic curve to be altered. In addition, the selection of the appropriate polynomial and the application of the coefficients of its inverse to the amplifier output characteristic curve should be done very quickly, or as close to real-time as possible.
Slow memory effect is defined as changes to the amplifier output characteristics due to aging, slow changes in ambient temperature, humidity, etc. Fast memory effect is defined as changes to the amplifier output characteristics due to instantaneous changes in the operating temperature of the amplifier. Because of these changes, it is generally not sufficient to use the same inverse polynomial coefficients all of the time. FIGS. 3A and 3B are graphs illustrating the amplifier output characteristic curves for various temperatures for amplitude and phase, respectively. The line 11 in FIG. 3 represents the target gain, which corresponds to the ideal amplifier output characteristic for amplitude. Curves 12-16 in FIG. 3 are the amplifier amplitude output characteristic curves for operating temperatures T1-T5, respectively, where T5<T4<T3<T2<T1. The amplifier curves 12-16 as a function of temperature are commonly referred to as memory characteristic curves.
It is known to use adaptive negative feedback systems that measure the amplifier output and determine which polynomial coefficients to select based on the output of the amplifier. In such systems, based on the measured amplifier output, it is determined which amplifier memory characteristic curve corresponds to the current temperature, and the corresponding coefficients are applied to correct for gain and phase (delta coefficients for gain and distortion coefficients for phase). One disadvantage of these systems is that they typically use large arrays of read-only memory (ROM) or complex lookup tables to store the coefficients and complex computer processing, which increases power consumption and adds delay in the feedback loop. Also, such systems often use expensive temperature sensors to sense the amplifier temperature to determine which coefficients to select. Sensing amplifier temperature not only increases system costs, but also makes the system prone to error. Furthermore, such systems are incapable of performing fast memory compensation using coefficients that correspond to an appropriate memory characteristic curve. Rather, a composite curve is typically used, which blends all temperatures into a resultant curve. Using the resultant curve rather than individual memory curves also makes the system more error prone.
Accordingly, a need exists for an amplifier digital pre-distorter that does not require large memory arrays for storing the delta and distortion coefficients, that does not require temperature sensors, and that is capable of performing fast memory compensation without the need to use a resultant memory characteristic curve.